The present invention is directed to a high pressure ultra-filtration (HPUF) installation which allows utilizing ultra-filtration pressures as high as 100 bars at least due to the use of suitable pressure tight diaphragms.
According to the present state of the art, ultrafiltration diaphragms operate in the pressure range of up to 10 bars and the service pressure of the great majority of the installations is limited to less than 4 bars.
The reason for this is that plastic diaphragms of different basic materials are almost exclusively used, for example a cellulose acetate or polyamide, in the form of asymmetrical diaphragms. The term "asymmetrical diaphragms" is used to denote diaphragms with a thin active layer, which is decisive in the separation of materials, with a relatively thick and highly porous "back-up" layer. The mechanical strength of the diaphragm is so small that pressures in excess of the above cited pressures cannot be resisted. As the pressure across a diaphragm increases, the porous structure of the diaphragm changes together with the separating or filtering characteristics. Admittedly, the porous structure ensures a high exchange capacity, but it also results in limited mechanical strength.
In addition to the plastic diaphragms, a number of diaphragms made of graphite, which is an inorganic material, are also used. Though such diaphragms should be pressure stable up to 35 bars, the equipment in which they are fitted normally operates around 7 bars.
The main components of an ultra-filtration installation, apart from the regulator and control equipment, are pumps producing the ultra-filtration pressure and the diaphragm modules which contain the diaphragms and in which the ultra-filtrative material separation takes place. Depending on the manner in which the diaphragm material is arranged in the module, the following differentiations are made for different module structures. These module structures are grouped:
sheet membrans in "plate and frame" stack or spiral wound modules tubular membrans or tubular modules PA1 capillary modules of diaphragm capillaries which are often referred to as hollow fibers.
In plate and frame modules, the diaphragm foils are clamped between suitable backing slabs with flat membranes separating the inlet for the crude solution from the discharge or outlet for the permeate or filtrate. The backing slabs contain channels and profiles of the same or different dimensions over the entire face of the slab and these channels or profiles maintain a flow without allowing the formation of dead filtration areas.
In spiral wound modules two diaphragm foils of each type are coiled with a suitable backing web of supporting material and built into a modular tube. The inlet flow occurs through the backing web and through the two coil layers and the backing web ensures constant turbulence of the flow by constantly redirecting it. The permeate discharge occurs through the two diaphragm foils between which a further porous supporting insert is frequently wound. Tubular modules, consist of tubes with a porous drainage internal layer upon which a prefabricated diaphragm hose complex is placed or is applied directly to the diaphragm hose. The crude solution passes through the tube having a diameter in the order of centimeters and the permeate or filtrate which passes through the diaphragm is discharged through the drainage layer.
In capillary and hollow fiber modules, the capillaries or hollow fibers are bonded at the ends into a jacket using a suitable adhesive. The crude solution passes through the capillaries and the permeate or filtrate that passes through the diaphragms flows into the outer chamber.
As in all convective material exchange problems, the transfer through the diaphragm is improved when a turbulent flow is ensured. The powerful turbulent solution transport passing transversely to the flow direction carries the solution more vigorously to the diaphragms on the one hand and also ensures a good removal of the retained components on the other hand. The retained component will inhibit the permeation of the diaphragm either due to concentration build up or deposits on the diaphragm.
With the exception of tubular modules in most cases, none of the present installation operate with a turbulent flow. In many of the above cited modular designs, it is not possible to achieve a turbulent flow owing to the design and surface parameters. In some cases, a turbulent effect is simulated, for instance, by turbulence of the crude solution flow as a result of passing it through the backing web of the wound module or by introducing mixed components or turbulence promoters, for instance, in the tubular module. Nevertheless, this does not produce a genuine flow turbulence.